1. Field of the Invention
The present invention relates generally to the field of optics, and more specifically, to a multi-aperture telescope with high fill factor. In a spaced-based telescope application of the invention, light from a plurality of collector telescopes is combined by a combiner telescope located at a real exit pupil of the collector telescopes, thus allowing a fill factor for a multiple aperture telescope array to be comparable to a segmented primary telescope.
2. Description of the Related Art
Traditionally a telescope system consists of a large collecting element (usually called the “primary” which may be either a lens or a mirror) located at the entrance pupil and possibly some smaller elements to relay or convey the light to an image plane. A refractive example of this is shown in FIG. 1, which shows a relatively large aperture telescope with relay elements. In particular, the telescope includes a primary lens 10 near the entrance pupil 12, and two smaller relay lenses 14 and 16. In the example of FIG. 1, the light from an intermediate image formed at the intermediate image plane 18 is first collimated and then relayed in such a way that a real exit pupil 20, i.e., an image of the entrance pupil, is formed before the image plane 22.
As telescope systems become larger and larger, in order to achieve higher resolution and to collect more light, a point is eventually reached where the size of the required elements, especially the primary minor, exceeds the current state of the art in fabrication and support. For telescopes larger than this, the entrance pupil must either be divided into manageable segments, so as to provide a “segmented primary” approach, or the entrance pupil is divided into an array of separate telescopes, providing a “multiple telescope” approach.
An example of a multiple telescope array is shown in FIG. 2, which shows the use of refractive elements for simplicity. In reality, most if not all large telescopes use reflective elements. As seen in FIG. 2, a first sub-aperture telescope includes a primary element 24 positioned near an entrance pupil 26, and a secondary element 28. A second sub-aperture telescope similarly includes a primary element 30 and a secondary element 32. The light from the sub-aperture telescopes is recombined coherently with additional optics, such as flat mirrors generally referred to by the numeral 34, to form an image, at the image plane 36, with high resolution. Lens element 38, disposed before the image of the exit pupil, is part of the combiner optics.
Essentially, the structure shown in FIG. 2 provides a sparse array telescope having a plurality of sub-aperture afocal telescopes and a combiner telescope. The manner in which the light from the sub-apertures is recombined is critical for good resolution. U.S. Pat. No. 5,905,591 to Duncan et al. discloses a particularly effective way of doing so for a space-based telescope in which a plurality of sub-aperture telescopes are disposed on deployable booms. The portion of the light falling on the inscribed entrance pupil that is collected by the sub-aperture array is referred to as the “fill factor.” With the structure shown in U.S. Pat. No. 5,905,591, the recombining scheme would have a fill factor of much less than 50% (assuming a finite field of view). Low fill factors such as this are associated with designs that are said to be “sparse.”
A need exists for an optical system that is capable of achieving fill factors greater than about 50%, thus constituting a “high” fill factor, or a “highly filled” telescope.
Of course, the present invention relates to space-based telescopes, which have problems uniquely associated with devices that must be launched and precisely controlled. Astronomical instruments have been placed in space since the early days of space exploration.
What makes a space-based telescope desirable is the visual clarity that results from being above the Earth's atmosphere. However, location above the atmosphere is only part of the equation, and like any terrestrial telescope, an excellent optical system is required to achieve its resolution potential. The HST includes an optical telescope assembly having a primary mirror and a secondary mirror, arranged in a Ritchey-Chretien Cassegrain configuration, in which the two mirrors form a focused image over the largest possible field of view. In particular, incoming light travels down a tube fitted with baffles that keep out stray light. The light is collected by the concave primary mirror and reflected toward the smaller, convex secondary mirror. The secondary mirror reflects the light back toward a hole in the center of the primary. The light is then focused on a small area called the focal plane, where it is transferred to various instruments.
Because it is difficult to launch and package payloads of large diameter, telescopes having multiple, relatively smaller primary reflectors have been devised. The aforementioned U.S. Pat. No. 5,905,591 to Duncan et al. describes an optical system, which provides a greater diameter by using sub-aperture telescopes that are mounted on a deployable boom. Such arrangements require combining relay optics as noted above.
Others have recognized the problems of launching a large aperture telescope. U.S. Pat. No. 5,898,529 to Meyer et al. describes a space-based telescope in which a primary reflector is segmented to provide two concentric rows of reflector segments, thus rendering the telescope stowable in a disposition more suitable for launching from a cargo bay of limited dimensions. However, this approach is to provide a single telescope rather than an array of telescopes.